The present invention relates to an exhaust gas recirculation system for controlling the flow of exhaust gas from an exhaust gas passage to an engine intake passage of an internal combustion engine. More particularly, the present invention relates to an exhaust gas recirculation system using pressure-based feedback control.
Exhaust gas recirculation (EGR) systems are employed in automotive vehicles in order to help reduce various engine emissions. Such systems typically employ an EGR valve that is disposed between the engine exhaust manifold and the engine intake manifold, and operable, when in an open position, to recirculate a portion of the exhaust gases from the exhaust side of the engine back to the intake side.
An actuator is employed for moving the EGR valve between its open and closed positions, with the recirculation of exhaust gas being more appropriate at certain times. Typically, the appropriate time for recirculation is when the vehicle is traveling at lower speeds, when the pressure at the intake side of the engine is less than the pressure at the exhaust side of the engine.
Many prior art EGR systems employ air pressure as an actuator. However, in order to achieve more precise control, electrically-actuated EGR valves have also been introduced. Electrically-actuated EGR valve systems may employ software-implemented control logic which implements open or closed loop control. Such control logic controls current to an electric actuator motor which, in turn, positions the EGR valve. In such systems, the control logic may generate pulse width modulated (PWM) signals to power the actuator motor, and modulate the acceleration and deceleration of the EGR valve as it moves to its desired positions.
The open loop system is generally faster and less expensive than a closed loop system, but requires a separate device to diagnose failure. This other device is usually a manifold absolute pressure (MAP) sensor. The cost of the MAP sensor offsets much of the cost benefit of the open loop system. Open loop EGR systems typically use a stepper motor valve which reliably moves the valve to a requested position. Flow through the valve is inferred by knowing the pressure before and after the valve in concert with its position. Unfortunately, open loop flow prediction degrades rapidly as particulates clog the valve, and requires the valve itself to have minimal variability in its manufactured flow characteristics.
Closed loop systems measure flow using pressures before and after a control orifice, which is located within the EGR flow path. The measured flow is compared to the requested flow. The valve is then moved to minimize flow error. This process has improved steady state performance in terms of the actual flow matching the requested flow, regardless of how degraded or variable the valve flow characteristic may be. However, this system is slower in reaching its final position. Furthermore, if used too aggressively to minimize the time response, closed loop systems can be unstable.
With the advent of more aggressive nitrous oxide emission standards, federal legislation requires that a malfunction indicator light be activated when the tailpipe nitrous oxide level exceeds 1.5 times the legislated standard. Unfortunately, prior art systems have experienced difficulty in providing this feature. Because of the low signal to noise ratio when comparing EGR % to manifold absolute pressure, prior art system can not meet these requirements.
The disadvantages associated with these conventional EGR system control techniques have made it apparent that a new technique for EGR system control is needed. The new technique should not degrade over time or require minimum variability in existing EGR valves. Additionally, the new technique should have a minimum response time without stability problems. The present invention is directed to these ends.
It is, therefore, an object of the present invention to provide an improved and reliable EGR system using pressure-based feedback control. Another object of the present invention is to prevent system degradation due to EGR valve conditions. An additional object of the present invention is to have low response time without going unstable.
In accordance with the objects of this invention, an EGR system using pressure-based feedback control is provided. In one embodiment of the invention, an EGR system using pressure-based feedback control includes an actuator for moving an EGR valve between a closed position and an open position, and a position sensor for detecting the position of the EGR valve and generating a position output signal. A processing circuit receives the position output signal and transmits electrical control signals to energize the actuator and move the EGR valve. A controller includes control logic operative to determine the position for the EGR valve using open loop control during transient behavior and closed loop control for steady state accuracy.
The present invention thus achieves an improved EGR system using pressure-based feedback control. The present invention is advantageous in that the system has a response time equivalent to an open loop system while maintaining steady state performance. Additionally, the present invention provides hardware and software to more robustly measure the pressure drop across an orifice in the EGR flow stream.
Additional advantages and features of the present invention will become apparent from the description that follows, and may be realized by means of the instrumentalities and combinations particularly pointed out in the appended claims, taken in conjunction with the accompanying drawings.